This invention relates to blood pumps to be implanted in a patient for supporting the patient's heart. In particular, the blood pump may be used as a “bridge to recovery” device, whereby the blood pump temporarily supports the patient's heart until it has sufficiently recovered.
Blood pumps of different types are known, such as axial blood pumps, centrifugal blood pumps or mixed type blood pumps, where the blood flow is caused by both axial and radial forces. Blood pumps may be inserted into a patient's vessel such as the aorta by means of a catheter, or may be placed in the thoracic cavity. A blood pump typically comprises a pump casing having a blood flow inlet and a blood flow outlet connected by a passage. In order to cause a blood flow along the passage from the blood flow inlet to the blood flow outlet an impeller is rotatably supported within the pump casing, with the impeller being provided with blades for conveying blood.
The impeller is supported within the pump casing by means of at least one bearing, which may be of different types depending on the intended use of the blood pump, for instance whether the blood pump is intended only for short term use (some hours or some days) or long term use (weeks or years). A variety of bearings are known, such as contact-type bearings and non-contact bearings. In non-contact bearings the bearing surfaces do not contact each other, e.g. in magnetic bearings, in which the bearing surface “levitate” due to repelling magnetic forces. Generally, contact-type bearings may include all types of bearings, in which the bearing surfaces may contact at least partially during operation of the pump at any time (i.e. always or intermittently), e.g. in slide bearings, pivot bearings, hydrodynamic bearings, hydrostatic bearings, ball bearings etc. or any combination thereof. In particular, contact-type bearings may be “blood immersed bearings”, where the bearing surfaces have blood contact. Contact-type bearings may heat up during use and are subject to mechanical wear caused by the contact of the rotating bearing surface and the static bearing surface during operating of the pump. It is desirable to supply a cooling fluid to the bearing, such as the blood itself. In non-contact-type bearings, the bearing surfaces do not have physical contact but are spaced by a clearance, which is in fluid connection with the passage or other fluid supply. Likewise, other clearances between the impeller and the pump casing should be washed out to avoid blood clotting and clogging, for instance at the downstream front face of the impeller.
Arrangements for rinsing clearances or bearing surfaces within a blood pump are disclosed for instance in US 2011/0238172 A1 (now U.S. Pat. No. 9,616,157). Wash out channels extend through the impeller and are in fluid communication with the passage and the clearance via first and second openings. The pressure distribution in the pump casing, where the pressure increases along the impeller in a downstream direction, gives rise to a blood flow through the clearance and the wash out channel, the blood entering the clearance at a downstream end of the impeller and flowing through the wash out channel towards an area of the passage with lower pressure. This wash out flow has the disadvantage of depending on the rotational speed of the impeller, because a pressure difference must be created to cause the blood flow. Other forces such as centrifugal forces and a counter force due to the backward direction of the wash out flow also have to be overcome. In another embodiment disclosed in US 2011/0238172 A1 (now U.S. Pat. No. 9,616,157), in which the impeller is supported in the pump casing by a hydrodynamic bearing, the inlet opening of the wash out channel is disposed at the upstream end of the impeller. Secondary blades at the downstream end of the impeller are provided to cause a wash out flow through the wash out channel and the clearance in a direction from an area of low pressure to an area of higher pressure.